Interrogator/responder systems are known that are adapted for use with a plurality of mobile objects such as vehicles and that function to communicate information from one of a plurality of spaced-apart locations to one of the mobile objects when the mobile object is in proximity to the location. In a typical interrogation/responder system designed for use with a transportation system, each vehicle is provided with an interrogator unit and a responder unit is positioned at each location. When one of the vehicles comes into proximity to one of the locations, an interrogating signal from the interrogator unit causes the responder unit to transmit a response signal to the interrogator unit, which response signal includes selected information such as the identity of the responder unit location. The information within the response signal is stored within the interrogator unit and may be transmitted to a central location along with similar information from the interrogator units of the other vehicles so as to provide a centralized record and display of vehicle location information which can be used to control vehicle movements in the transportation system.
In such a transportation system applications, a larger number of responder units are required and each responder unit must be capable of operating for extended periods of time without requiring servicing. It is therefore important that each responder unit be inexpensive to construct and consume as little power as possible. To meet the requirements of low cost and low power consumption, the responder unit is advantageously powered from the interrogating signal, as disclosed in U.S. Pat. No. 3,989,619, Object Location/Identification System, Carsten et al., issued Aug. 5, 1975 to the assignee of the present invention.
Due to the fact that the responder unit is powered by the interrogating signal, the range of information communication between the responder unit and the interrogator unit is extremely limited, e.g., a few meters. This range limitation on interrogator/responder systems of the type described makes such systems inherently unsuitable for use in applications in which information is to be communicated from a specific location to a plurality of vehicles in proximity to the location. An exemplary application of the latter type would be an open-pit mine control system in which a plurality of trucks are transporting ore from one or more loading locations to one or more discharge locations. At any given point in time, a number of trucks may be waiting at one of the locations and an interrogator/responder system would be capable of providing information as to the identity of that location only to the truck whose interrogator unit was within the limited range of the responder unit at the location. As a consequence, the remaining trucks at the location would not be receiving information as to the identity of the location, so that the centralized record and display at the central location would not truly reflect the actual location of all trucks in the system.
The loss of communications (and thus of vehicle location information) that is caused by the range limitation of interrogator/responder systems could be overcome by the use of a conventional VHF beacon system. In such a system, each location would be equipped with a beacon that transmitted, asynchronously with respect to any other beacon, a periodic VHF signal including information as to the identity of the location. Each vehicle would then be provided with a VHF receiver, and the amplitude characteristics of the VHF signals from the beacons and the design of each receiver would be chosen so that a VHF signal from a given beacon would be detected by a vehicle's receiver only when the vehicle was within a predetermined range of a location. Such a beacon system is subject, however, to the serious disadvantage that the predetermined range of beacon detection cannot be precisely controlled due to the fact that the signal from each beacon is at VHF. In the open pit mine control system application previously discussed, the locations are close enough to each other so that the receiver on a vehicle would in certain circumstances receive and detect VHF signals from two or more beacons even though the vehicle was closer to one of the beacons than to the others. In such circumstances, loss of accurate vehicle location information would result.
It is therefore an object of this invention to provide an improved information system for a plurality of mobile objects.
It is another object of this invention to provide such an information system that is operative to communicate information from a specific location to one of the objects only when the object is within a predetermined range of the location.
It is yet another object of this invention to provide such a system which is capable of communicating information from a location to a plurality of objects within the predetermined range of the location.
It is still another object of this invention to provide such a system which is capable of more precisely controlling the predetermined range of information communication than prior art VHF beacon systems.
It is a further object of this invention to provide such a system in which the predetermined range of information communication is substantially greater than that achieved with prior art interrogator/responder systems.
It is yet a further object of this invention to provide such a system in which the portion of the system at each location has relatively low power consumption.